TSVWG Working Group G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Best Current Practice April 04, 2016
Expires: October 6, 2016
Network Transport Circuit Breakersdraft-ietf-tsvwg-circuit-breaker-15
Abstract
This document explains what is meant by the term "network transport
Circuit Breaker" (CB). It describes the need for circuit breakers
for network tunnels and applications when using non-congestion-
controlled traffic, and explains where circuit breakers are, and are
not, needed. It also defines requirements for building a circuit
breaker and the expected outcomes of using a circuit breaker within
the Internet.
Status of This Memo
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This Internet-Draft will expire on October 6, 2016.
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In networking, the Circuit Breaker (CB) principle can be used as a
protection mechanism of last resort to avoid persistent excessive
congestion impacting other flows that share network capacity.
Persistent congestion was a feature of the early Internet of the
1980s. This resulted in excess traffic starving other connections
from access to the Internet. It was countered by the requirement to
use congestion control (CC) in the Transmission Control Protocol
(TCP) [Jacobsen88]. These mechanisms operate in Internet hosts to
cause TCP connections to "back off" during congestion. The addition
of a congestion control to TCP (currently documented in [RFC5681]
ensured the stability of the Internet, because it was able to detect
congestion and promptly react. This was effective in an Internet
where most TCP flows were long-lived (ensuring that they could detect
and respond to congestion before the flows terminated). Although TCP
was by far the dominant traffic, this is no longer the always the
case, and non-congestion-controlled traffic, including many
applications using the User Datagram Protocol (UDP), can form a
significant proportion of the total traffic traversing a link. The
current Internet therefore requires that non-congestion-controlled
traffic is considered to avoid persistent excessive congestion.
A network transport Circuit Breaker is an automatic mechanism that is
used to continuously monitor a flow or aggregate set of flows. The
mechanism seeks to detect when the flow(s) experience persistent
excessive congestion. When this is detected, a Circuit Breaker
terminates (or significantly reduce the rate of) the flow(s). This
is a safety measure to prevent starvation of network resources
denying other flows from access to the Internet. Such measures are
essential for an Internet that is heterogeneous and for traffic that
is hard to predict in advance. Avoiding persistent excessive
congestion is important to reduce the potential for "Congestion
Collapse" [RFC2914].
There are important differences between a transport Circuit Breaker
and a congestion control method. Congestion control (as implemented
in TCP, SCTP, and DCCP) operates on a timescale on the order of a
packet round-trip-time (RTT), the time from sender to destination and
return. Congestion at a network link can also be detected using
Explicit Congestion Notification (ECN) [RFC3168], which allows the
network to signal congestion by marking ECN-capable packets with a
Congestion Experienced (CE) mark. Both loss and reception of CE-
marked packets are treated as congestion events. Congestion control
methods are able to react to a congestion event by continuously
adapting to reduce their transmission rate. The goal is usually to
limit the transmission rate to a maximum rate that reflects a fair
use of the available capacity across a network path. These methods
typically operate on individual traffic flows (e.g., a 5-tuple that
includes the IP addresses, protocol, and ports).
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In contrast, Circuit Breakers are recommended for non-congestion-
controlled Internet flows and for traffic aggregates, e.g., traffic
sent using a network tunnel. They operate on timescales much longer
than the packet RTT, and trigger under situations of abnormal
(excessive) congestion. People have been implementing what this
document characterizes as circuit breakers on an ad hoc basis to
protect Internet traffic. This document therefore provides guidance
on how to deploy and use these mechanisms. Later sections provide
examples of cases where circuit-breakers may or may not be desirable.
A Circuit Breaker needs to measure (meter) some portion of the
traffic to determine if the network is experiencing congestion and
needs to be designed to trigger robustly when there is persistent
excessive congestion.
A Circuit Breaker trigger will often utilize a series of successive
sample measurements metered at an ingress point and an egress point
(either of which could be a transport endpoint). The trigger needs
to operate on a timescale much longer than the path round trip time
(e.g., seconds to possibly many tens of seconds). This longer period
is needed to provide sufficient time for transport congestion control
(or applications) to adjust their rate following congestion, and for
the network load to stabilize after any adjustment. Congestion
events can be common when a congestion-controlled transport is used
over a network link operating near capacity. Each event results in
reduction in the rate of the transport flow experiencing congestion.
The longer period seeks to ensure that a Circuit Breaker does not
accidentally trigger following a single (or even successive)
congestion events.
Once triggered, the Circuit Breaker needs to provide a control
function (called the "reaction"). This removes traffic from the
network, either by disabling the flow or by significantly reducing
the level of traffic. This reaction provides the required protection
to prevent persistent excessive congestion being experienced by other
flows that share the congested part of the network path.
Section 4 defines requirements for building a Circuit Breaker.
The operational conditions that cause a Circuit Breaker to trigger
ought to be regarded as abnormal. Examples of situations that could
trigger a Circuit Breaker include:
o anomalous traffic that exceeds the provisioned capacity (or whose
traffic characteristics exceed the threshold configured for the
Circuit Breaker);
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o traffic generated by an application at a time when the provisioned
network capacity is being utilised for other purposes;
o routing changes that cause additional traffic to start using the
path monitored by the Circuit Breaker;
o misconfiguration of a service/network device where the capacity
available is insufficient to support the current traffic
aggregate;
o misconfiguration of an admission controller or traffic policer
that allows more traffic than expected across the path monitored
by the Circuit Breaker.
Other mechanisms could also be available to network operators to
detect excessive congestion (e.g., an observation of excessive
utilisation for a port on a network device). Utilising such
information, operational mechanisms could react to reduce network
load over a shorter timescale than those of a network transport
Circuit Breaker. The role of the Circuit Breaker over such paths
remains as a method of last resort. Because it acts over a longer
timescale, the Circuit Breaker ought to trigger only when other
reactions did not succeed in reducing persistent excessive
congestion.
In many cases, the reason for triggering a Circuit Breaker will not
be evident to the source of the traffic (user, application, endpoint,
etc). A Circuit Breaker can be used to limit traffic from
applications that are unable, or choose not, to use congestion
control, or where the congestion control properties of the traffic
cannot be relied upon (e.g., traffic carried over a network tunnel).
In such circumstances, it is all but impossible for the Circuit
Breaker to signal back to the impacted applications. In some cases
applications could therefore have difficulty in determining that a
Circuit Breaker has triggered, and where in the network this
happened.
Application developers are therefore advised, where possible, to
deploy appropriate congestion control mechanisms. An application
that uses congestion control will be aware of congestion events in
the network. This allows it to regulate the network load under
congestion, and is expected to avoid triggering a network Circuit
Breaker. For applications that can generate elastic traffic, this
will often be a preferred solution.
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Internet-Draft April 20161.1. Types of Circuit Breaker
There are various forms of network transport circuit breaker. These
are differentiated mainly on the timescale over which they are
triggered, but also in the intended protection they offer:
o Fast-Trip Circuit Breakers: The relatively short timescale used by
this form of circuit breaker is intended to provide protection for
network traffic from a single flow or related group of flows.
o Slow-Trip Circuit Breakers: This circuit breaker utilizes a longer
timescale and is designed to protect network traffic from
congestion by traffic aggregates.
o Managed Circuit Breakers: Utilize the operations and management
functions that might be present in a managed service to implement
a circuit breaker.
Examples of each type of circuit breaker are provided in section 4.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Design of a Circuit-Breaker (What makes a good circuit breaker?)
Although circuit breakers have been talked about in the IETF for many
years, there has not yet been guidance on the cases where circuit
breakers are needed or upon the design of circuit breaker mechanisms.
This document seeks to offer advice on these two topics.
Circuit Breakers are RECOMMENDED for IETF protocols and tunnels that
carry non-congestion-controlled Internet flows and for traffic
aggregates. This includes traffic sent using a network tunnel.
Designers of other protocols and tunnel encapsulations also ought to
consider the use of these techniques as a last resort to protect
traffic that shares the network path being used.
This document defines the requirements for design of a Circuit
Breaker and provides examples of how a Circuit Breaker can be
constructed. The specifications of individual protocols and tunnel
encapsulations need to detail the protocol mechanisms needed to
implement a Circuit Breaker.
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and measurement functions could alternatively be located at other
locations (e.g., at a network operations centre.)
In the context of a Circuit Breaker, the ingress and egress functions
could be implemented in different places. For example, they could be
located in network devices at a tunnel ingress and at the tunnel
egress. In some cases, they could be located at one or both network
endpoints (see figure 2), implemented as components within a
transport protocol.
+----------+ +----------+
| Ingress | +-+ +-+ +-+ | Egress |
| Endpoint +->+R+--+R+--+R+--+ Endpoint |
+--+----+--+ +-+ +-+ +-+ +----+-----+
^ | |
| +--+------+ +----+----+
| | Ingress | | Egress |
| | Meter | | Meter |
| +----+----+ +----+----+
| | |
| +--- +----+ |
| | Measure +<-----------------+
| +----+----+ Reported
| | Egress
| +----+----+ Measurement
| | Trigger |
| +----+----+
| |
+---<--+
Reaction
Figure 2: An endpoint CB implemented at the sender (ingress) and
receiver (egress).
The set of components needed to implement a Circuit Breaker are:
1. An ingress meter (at the sender or tunnel ingress) that records
the number of packets/bytes sent in each measurement interval.
This measures the offered network load for a flow or set of
flows. For example, the measurement interval could be many
seconds (or every few tens of seconds or a series of successive
shorter measurements that are combined by the Circuit Breaker
Measurement function).
2. An egress meter (at the receiver or tunnel egress) that records
the number/bytes received in each measurement interval. This
measures the supported load for the flow or set of flows, and
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could utilize other signals to detect the effect of congestion
(e.g., loss/congestion marking [RFC3168] experienced over the
path). The measurements at the egress could be synchronised
(including an offset for the time of flight of the data, or
referencing the measurements to a particular packet) to ensure
any counters refer to the same span of packets.
3. A method that communicates the measured values at the ingress and
egress to the Circuit Breaker Measurement function. This could
use several methods including: Sending return measurement packets
(or control messages) from a receiver to a trigger function at
the sender; an implementation using Operations, Administration
and Management (OAM); or sending an in-band signalling datagram
to the trigger function. This could also be implemented purely
as a control plane function, e.g., using a software-defined
network controller.
4. A measurement function that combines the ingress and egress
measurements to assess the present level of network congestion.
(For example, the loss rate for each measurement interval could
be deduced from calculating the difference between ingress and
egress counter values.) Note the method does not require high
accuracy for the period of the measurement interval (or therefore
the measured value, since isolated and/or infrequent loss events
need to be disregarded.)
5. A trigger function that determines whether the measurements
indicate persistent excessive congestion. This function defines
an appropriate threshold for determining that there is persistent
excessive congestion between the ingress and egress. This
preferably considers a rate or ratio, rather than an absolute
value (e.g., more than 10% loss, but other methods could also be
based on the rate of transmission as well as the loss rate). The
Circuit Breaker is triggered when the threshold is exceeded in
multiple measurement intervals (e.g., 3 successive measurements).
Designs need to be robust so that single or spurious events do
not trigger a reaction.
6. A reaction that is applied at the Ingress when the Circuit
Breaker is triggered. This seeks to automatically remove the
traffic causing persistent excessive congestion.
7. A feedback control mechanism that triggers when either the
receive or ingress and egress measurements are not available,
since this also could indicate a loss of control packets (also a
symptom of heavy congestion or inability to control the load).
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In this figure, each endpoint includes a meter that performs a local
egress load measurement. An endpoint also extracts the received
ingress measurement from the traffic, and compares the ingress and
egress measurements to determine if the Circuit Breaker ought to be
triggered. This measurement has to be robust to loss (see previous
section). If the Circuit Breaker is triggered, it generates a
multicast leave message for the egress (e.g., an IGMP or MLD message
sent to the last hop router), which causes the upstream router to
cease forwarding traffic to the egress endpoint [RFC1112].
Any multicast router that has no active receivers for a particular
multicast group will prune traffic for that group, sending a prune
message to its upstream router. This starts the process of releasing
the capacity used by the traffic and is a standard multicast routing
function (e.g., using Protocol Independent Multicast Sparse Mode
(PIM-SM) routing protocol [RFC4601]). Each egress operates
autonomously, and the Circuit Breaker "reaction" is executed by the
multicast control plane (e.g., by PIM) requiring no explicit
signalling by the Circuit Breaker along the communication path used
for the control messages. Note: there is no direct communication
with the Ingress, and hence a triggered Circuit Breaker only controls
traffic downstream of the first hop multicast router. It does not
stop traffic flowing from the sender to the first hop router; this is
common practice for multicast deployment.
The method could also be used with a multicast tunnel or subnetwork
(e.g., Section 5.2, Section 5.3), where a meter at the ingress
generates additional control messages to carry the measurement data
towards the egress where the egress metering is implemented.
3.2.2. Use with control protocols supporting pre-provisioned capacity
Some paths are provisioned using a control protocol, e.g., flows
provisioned using the Multi-Protocol Label Switching (MPLS) services,
paths provisioned using the resource reservation protocol (RSVP),
networks utilizing Software Defined Network (SDN) functions, or
admission-controlled Differentiated Services. Figure 1 shows one
expected use case, where in this usage a separate device could be
used to perform the measurement and trigger functions. The reaction
generated by the trigger could take the form of a network control
message sent to the ingress and/or other network elements causing
these elements to react to the Circuit Breaker. Examples of this
type of use are provided in section Section 5.3.
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Internet-Draft April 20163.2.3. Unidirectional Circuit Breakers over Controlled Paths
A Circuit Breaker can be used to control uni-directional UDP traffic,
providing that there is a communication path that can be used for
control messages to connect the functional components at the Ingress
and Egress. This communication path for the control messages can
exist in networks for which the traffic flow is purely
unidirectional. For example, a multicast stream that sends packets
across an Internet path and can use multicast routing to prune flows
to shed network load. Some other types of subnetwork also utilize
control protocols that can be used to control traffic flows.
4. Requirements for a Network Transport Circuit Breaker
The requirements for implementing a Circuit Breaker are:
1. There needs to be a communication path for control messages to
carry measurement data from the ingress meter and from the
egress meter to the point of measurement. (Requirements 16-18
relate to the transmission of control messages.)
2. A CB is REQUIRED to define a measurement period over which the
CB Measurement function measures the level of congestion or
loss. This method does not have to detect individual packet
loss, but MUST have a way to know that packets have been lost/
marked from the traffic flow.
3. An egress meter can also count ECN [RFC3168] congestion marks as
a part of measurement of congestion, but in this case, loss MUST
also be measured to provide a complete view of the level of
congestion. For tunnels,
[ID-ietf-tsvwg-tunnel-congestion-feedback] describes a way to
measure both loss and ECN-marking; these measurements could be
used on a relatively short timescale to drive a congestion
control response and/or aggregated over a longer timescale with
a higher trigger threshold to drive a CB. Subsequent bullet
items in this section discuss the necessity of using a longer
timescale and a higher trigger threshold.
4. The measurement period used by a CB Measurement function MUST be
longer than the time that current Congestion Control algorithms
need to reduce their rate following detection of congestion.
This is important because end-to-end Congestion Control
algorithms require at least one RTT to notify and adjust the
traffic when congestion is experienced, and congestion
bottlenecks can share traffic with a diverse range of RTTs. The
measurement period is therefore expected to be significantly
longer than the RTT experienced by the CB itself.
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5. If necessary, a CB MAY combine successive individual meter
samples from the ingress and egress to ensure observation of an
average measurement over a sufficiently long interval. (Note
when meter samples need to be combined, the combination needs to
reflect the sum of the individual sample counts divided by the
total time/volume over which the samples were measured.
Individual samples over different intervals can not be directly
combined to generate an average value.)
6. A CB MUST be constructed so that it does not trigger under light
or intermittent congestion (see requirements 7-9).
7. A CB is REQUIRED to define a threshold to determine whether the
measured congestion is considered excessive.
8. A CB is REQUIRED to define the triggering interval, defining the
period over which the trigger uses the collected measurements.
CBs need to trigger over a sufficiently long period to avoid
additionally penalizing flows with a long path RTT (e.g., many
path RTTs).
9. A CB MUST be robust to multiple congestion events. This usually
will define a number of measured persistent congestion events
per triggering period. For example, a CB MAY combine the
results of several measurement periods to determine if the CB is
triggered (e.g., it is triggered when persistent excessive
congestion is detected in 3 of the measurements within the
triggering interval).
10. The normal reaction to a trigger SHOULD disable all traffic that
contributed to congestion (otherwise, see requirements 11,12).
11. The reaction MUST be much more severe than that of a Congestion
Control algorithm (such as TCP's congestion control [RFC5681] or
TCP-Friendly Rate Control, TFRC [RFC5348]), because the CB
reacts to more persistent congestion and operates over longer
timescales (i.e., the overload condition will have persisted for
a longer time before the CB is triggered).
12. A reaction that results in a reduction SHOULD result in reducing
the traffic by at least an order of magnitude. A response that
achieves the reduction by terminating flows, rather than
randomly dropping packets, will often be more desirable to users
of the service. A CB that reduces the rate of a flow, MUST
continue to monitor the level of congestion and MUST further
react to reduce the rate if the CB is again triggered.
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13. The reaction to a triggered CB MUST continue for a period that
is at least the triggering interval. Operator intervention will
usually be required to restore a flow. If an automated response
is needed to reset the trigger, then this needs to not be
immediate. The design of an automated reset mechanism needs to
be sufficiently conservative that it does not adversely interact
with other mechanisms (including other CB algorithms that
control traffic over a common path). It SHOULD NOT perform an
automated reset when there is evidence of continued congestion.
14. A CB trigger SHOULD be regarded as an abnormal network event.
As such, this event SHOULD be logged. The measurements that
lead to triggering of the CB SHOULD also be logged.
15. The control communication needs to carry measurements
(requirement 1) and, in some uses, also needs to transmit
trigger messages to the ingress. This control communication may
be in-band or out-of-band. The use of in-band communication is
RECOMMENDED when either design would be possible. The preferred
CB design is one that triggers when it fails to receive
measurement reports that indicate an absence of congestion, in
contrast to relying on the successful transmission of a
"congested" signal back to the sender. (The feedback signal
could itself be lost under congestion).
in-Band: An in-band control method SHOULD assume that loss of
control messages is an indication of potential congestion on
the path, and repeated loss ought to cause the CB to be
triggered. This design has the advantage that it provides
fate-sharing of the traffic flow(s) and the control
communications. This fate-sharing property is weaker when
some or all of the measured traffic is sent using a path that
differs from the path taken by the control traffic (e.g.,
where traffic and control messages follow a different path
due to use of equal-cost multipath routing, traffic
engineering, or tunnels for specific types of traffic).
Out-of-Band: An out-of-band control method SHOULD NOT trigger
CB reaction when there is loss of control messages (e.g., a
loss of measurements). This avoids failure amplification/
propagation when the measurement and data paths fail
independently. A failure of an out-of-band communication
path SHOULD be regarded as abnormal network event and be
handled as appropriate for the network, e.g., this event
SHOULD be logged, and additional network operator action
might be appropriate, depending on the network and the
traffic involved.
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16. The control communication MUST be designed to be robust to
packet loss. A control message can be lost if there is a
failure of the communication path used for the control messages,
loss is likely to also be experienced during congestion/
overload. This does not imply that it is desirable to provide
reliable delivery (e.g., over TCP), since this can incur
additional delay in responding to congestion. Appropriate
mechanisms could be to duplicate control messages to provide
increased robustness to loss, or/and to regard a lack of control
traffic as an indication that excessive congestion could be
being experienced [ID-ietf-tsvwg-RFC5405.bis]. If control
messages traffic are sent over a shared path, it is RECOMMENDED
that this control traffic is prioritized to reduce the
probability of loss under congestion. Control traffic also
needs to be considered when provisioning a network that uses a
Circuit Breaker.
17. There are security requirements for the control communication
between endpoints and/or network devices (Section 7). The
authenticity of the source and integrity of the control messages
(measurements and triggers) MUST be protected from off-path
attacks. When there is a risk of on-path attack, a
cryptographic authentication mechanism for all control/
measurement messages is RECOMMENDED.
5. Examples of Circuit Breakers
There are multiple types of Circuit Breaker that could be defined for
use in different deployment cases. There could be cases where a flow
become controlled by multiple Circuit Breakers (e.g., when the
traffic of an end-to-end flow is carried in a tunnel within the
network). This section provides examples of different types of
Circuit Breaker:
5.1. A Fast-Trip Circuit Breaker
[RFC2309] discusses the dangers of congestion-unresponsive flows and
states that "all UDP-based streaming applications should incorporate
effective congestion avoidance mechanisms". Some applications do not
use a full-featured transport (TCP, SCTP, DCCP). These applications
(e.g., using UDP and its UDP-Lite variant) need to provide
appropriate congestion avoidance. Guidance for applications that do
not use congestion-controlled transports is provided in
[ID-ietf-tsvwg-RFC5405.bis]. Such mechanisms can be designed to
react on much shorter timescales than a Circuit Breaker, that only
observes a traffic envelope. Congestion control methods can also
interact with an application to more effectively control its sending
rate.
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A fast-trip Circuit Breaker is the most responsive form of Circuit
Breaker. It has a response time that is only slightly larger than
that of the traffic that it controls. It is suited to traffic with
well-understood characteristics (and could include one or more
trigger functions specifically tailored the type of traffic for which
it is designed). It is not suited to arbitrary network traffic and
could be unsuitable for traffic aggregates, since it could
prematurely trigger (e.g., when the combined traffic from multiple
congestion-controlled flows leads to short-term overload).
Although the mechanisms can be implemented in RTP-aware network
devices, these mechanisms are also suitable for implementation in
endpoints (e.g., as a part of the transport system) where they can
also compliment end-to-end congestion control methods. A shorter
response time enables these mechanisms to triggers before other forms
of Circuit Breaker (e.g., Circuit Breakers operating on traffic
aggregates at a point along the network path).
5.1.1. A Fast-Trip Circuit Breaker for RTP
A set of fast-trip Circuit Breaker methods have been specified for
use together by a Real-time Transport Protocol (RTP) flow using the
RTP/AVP Profile [RTP-CB]. It is expected that, in the absence of
severe congestion, all RTP applications running on best-effort IP
networks will be able to run without triggering these Circuit
Breakers. A fast-trip RTP Circuit Breaker is therefore implemented
as a fail-safe that when triggered will terminate RTP traffic.
The sending endpoint monitors reception of in-band RTP Control
Protocol (RTCP) reception report blocks, as contained in SR or RR
packets, that convey reception quality feedback information. This is
used to measure (congestion) loss, possibly in combination with ECN
[RFC6679].
The Circuit Breaker action (shutdown of the flow) is triggered when
any of the following trigger conditions are true:
1. An RTP Circuit Breaker triggers on reported lack of progress.
2. An RTP Circuit Breaker triggers when no receiver reports messages
are received.
3. An RTP Circuit Breaker triggers when the long-term RTP throughput
(over many RTTs) exceeds a hard upper limit determined by a
method that resembles TCP-Friendly Rate Control (TFRC).
4. An RTP Circuit Breaker includes the notion of Media Usability.
This Circuit Breaker is triggered when the quality of the
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transported media falls below some required minimum acceptable
quality.
5.2. A Slow-trip Circuit Breaker
A slow-trip Circuit Breaker could be implemented in an endpoint or
network device. This type of Circuit Breaker is much slower at
responding to congestion than a fast-trip Circuit Breaker. This is
expected to be more common.
One example where a slow-trip Circuit Breaker is needed is where
flows or traffic-aggregates use a tunnel or encapsulation and the
flows within the tunnel do not all support TCP-style congestion
control (e.g., TCP, SCTP, TFRC), see [ID-ietf-tsvwg-RFC5405.bis]
section 3.1.3. A use case is where tunnels are deployed in the
general Internet (rather than "controlled environments" within an
Internet service provider or enterprise network), especially when the
tunnel could need to cross a customer access router.
5.3. A Managed Circuit Breaker
A managed Circuit Breaker is implemented in the signalling protocol
or management plane that relates to the traffic aggregate being
controlled. This type of Circuit Breaker is typically applicable
when the deployment is within a "controlled environment".
A Circuit Breaker requires more than the ability to determine that a
network path is forwarding data, or to measure the rate of a path -
which are often normal network operational functions. There is an
additional need to determine a metric for congestion on the path and
to trigger a reaction when a threshold is crossed that indicates
persistent excessive congestion.
The control messages can use either in-band or out-of-band
communications.
5.3.1. A Managed Circuit Breaker for SAToP Pseudo-Wires
[RFC4553], SAToP Pseudo-Wires (PWE3), section 8 describes an example
of a managed Circuit Breaker for isochronous flows.
If such flows were to run over a pre-provisioned (e.g., Multi-
Protocol Label Switching, MPLS) infrastructure, then it could be
expected that the Pseudowire (PW) would not experience congestion,
because a flow is not expected to either increase (or decrease) their
rate. If, instead, PW traffic is multiplexed with other traffic over
the general Internet, it could experience congestion. [RFC4553]
states: "If SAToP PWs run over a PSN providing best-effort service,
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they SHOULD monitor packet loss in order to detect "severe
congestion". The currently recommended measurement period is 1
second, and the trigger operates when there are more than three
measured Severely Errored Seconds (SES) within a period. If such a
condition is detected, a SAToP PW ought to shut down bidirectionally
for some period of time...".
The concept was that when the packet loss ratio (congestion) level
increased above a threshold, the PW was by default disabled. This
use case considered fixed-rate transmission, where the PW had no
reasonable way to shed load.
The trigger needs to be set at the rate that the PW was likely to
experience a serious problem, possibly making the service non-
compliant. At this point, triggering the Circuit Breaker would
remove the traffic preventing undue impact on congestion-responsive
traffic (e.g., TCP). Part of the rationale, was that high loss
ratios typically indicated that something was "broken" and ought to
have already resulted in operator intervention, and therefore need to
trigger this intervention.
An operator-based response to triggering of a Circuit Breaker
provides an opportunity for other action to restore the service
quality, e.g., by shedding other loads or assigning additional
capacity, or to consciously avoid reacting to the trigger while
engineering a solution to the problem. This could require the
trigger function to send a control message to a third location (e.g.,
a network operations centre, NOC) that is responsible for operation
of the tunnel ingress, rather than the tunnel ingress itself.
5.3.2. A Managed Circuit Breaker for Pseudowires (PWs)
Pseudowires (PWs) [RFC3985] have become a common mechanism for
tunneling traffic, and could compete for network resources both with
other PWs and with non-PW traffic, such as TCP/IP flows.
[ID-ietf-pals-congcons] discusses congestion conditions that can
arise when PWs compete with elastic (i.e., congestion responsive)
network traffic (e.g, TCP traffic). Elastic PWs carrying IP traffic
(see [RFC4488]) do not raise major concerns because all of the
traffic involved responds, reducing the transmission rate when
network congestion is detected.
In contrast, inelastic PWs (e.g., a fixed bandwidth Time Division
Multiplex, TDM) [RFC4553] [RFC5086] [RFC5087]) have the potential to
harm congestion responsive traffic or to contribute to excessive
congestion because inelastic PWs do not adjust their transmission
rate in response to congestion. [ID-ietf-pals-congcons] analyses TDM
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PWs, with an initial conclusion that a TDM PW operating with a degree
of loss that could result in congestion-related problems is also
operating with a degree of loss that results in an unacceptable TDM
service. For that reason, the document suggests that a managed
Circuit Breaker that shuts down a PW when it persistently fails to
deliver acceptable TDM service is a useful means for addressing these
congestion concerns. (See Appendix A of [ID-ietf-pals-congcons] for
further discussion.)
6. Examples where circuit breakers may not be needed.
A Circuit Breaker is not required for a single congestion-controlled
flow using TCP, SCTP, TFRC, etc. In these cases, the congestion
control methods are already designed to prevent persistent excessive
congestion.
6.1. CBs over pre-provisioned Capacity
One common question is whether a Circuit Breaker is needed when a
tunnel is deployed in a private network with pre-provisioned
capacity.
In this case, compliant traffic that does not exceed the provisioned
capacity ought not to result in persistent congestion. A Circuit
Breaker will hence only be triggered when there is non-compliant
traffic. It could be argued that this event ought never to happen -
but it could also be argued that the Circuit Breaker equally ought
never to be triggered. If a Circuit Breaker were to be implemented,
it will provide an appropriate response if persistent congestion
occurs in an operational network.
Implementing a Circuit Breaker will not reduce the performance of the
flows, but in the event that persistent excessive congestion occurs
it protects network traffic that shares network capacity with these
flows. It also protects network traffic from a failure when Circuit
Breaker traffic is (re)routed to cause additional network load on a
non-pre-provisioned path.
6.2. CBs with tunnels carrying Congestion-Controlled Traffic
IP-based traffic is generally assumed to be congestion-controlled,
i.e., it is assumed that the transport protocols generating IP-based
traffic at the sender already employ mechanisms that are sufficient
to address congestion on the path. A question therefore arises when
people deploy a tunnel that is thought to only carry an aggregate of
TCP traffic (or traffic using some other congestion control method):
Is there advantage in this case in using a Circuit Breaker?
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TCP (and SCTP) traffic in a tunnel is expected to reduce the
transmission rate when network congestion is detected. Other
transports (e.g, using UDP) can employ mechanisms that are sufficient
to address congestion on the path [ID-ietf-tsvwg-RFC5405.bis].
However, even if the individual flows sharing a tunnel each implement
a congestion control mechanism, and individually reduce their
transmission rate when network congestion is detected, the overall
traffic resulting from the aggregate of the flows does not
necessarily avoid persistent congestion. For instance, most
congestion control mechanisms require long-lived flows to react to
reduce the rate of a flow. An aggregate of many short flows could
result in many flows terminating before they experience congestion.
It is also often impossible for a tunnel service provider to know
that the tunnel only contains congestion-controlled traffic (e.g.,
Inspecting packet headers might not be possible). Some IP-based
applications might not implement adequate mechanisms to address
congestion. The important thing to note is that if the aggregate of
the traffic does not result in persistent excessive congestion
(impacting other flows), then the Circuit Breaker will not trigger.
This is the expected case in this context - so implementing a Circuit
Breaker ought not to reduce performance of the tunnel, but in the
event that persistent excessive congestion occurs the Circuit Breaker
protects other network traffic that shares capacity with the tunnel
traffic.
6.3. CBs with Uni-directional Traffic and no Control Path
A one-way forwarding path could have no associated communication path
for sending control messages, and therefore cannot be controlled
using a Circuit Breaker (compare with Section 3.2.3).
A one-way service could be provided using a path with dedicated pre-
provisioned capacity that is not shared with other elastic Internet
flows (i.e., flows that vary their rate). A forwarding path could
also be shared with other flows. One way to mitigate the impact of
traffic on the other flows is to manage the traffic envelope by using
ingress policing. Supporting this type of traffic in the general
Internet requires operator monitoring to detect and respond to
persistent excessive congestion.
7. Security Considerations
All Circuit Breaker mechanisms rely upon coordination between the
ingress and egress meters and communication with the trigger
function. This is usually achieved by passing network control
information (or protocol messages) across the network. Timely
operation of a Circuit Breaker depends on the choice of measurement
period. If the receiver has an interval that is overly long, then
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the responsiveness of the Circuit Breaker decreases. This impacts
the ability of the Circuit Breaker to detect and react to congestion.
If the interval is too short the Circuit Breaker could trigger
prematurely resulting in insufficient time for other mechanisms to
act, potentially resulting in unnecessary disruption to the service.
A Circuit Breaker could potentially be exploited by an attacker to
mount a Denial of Service (DoS) attack against the traffic being
controlled by the Circuit Breaker. Mechanisms therefore need to be
implemented to prevent attacks on the network control information
that would result in DoS.
The authenticity of the source and integrity of the control messages
(measurements and triggers) MUST be protected from off-path attacks.
Without protection, it could be trivial for an attacker to inject
fake or modified control/measurement messages (e.g., indicating high
packet loss rates) causing a Circuit Breaker to trigger and to
therefore mount a DoS attack that disrupts a flow.
Simple protection can be provided by using a randomized source port,
or equivalent field in the packet header (such as the RTP SSRC value
and the RTP sequence number) expected not to be known to an off-path
attacker. Stronger protection can be achieved using a secure
authentication protocol to mitigate this concern.
An attack on the control messages is relatively easy for an attacker
on the control path when the messages are neither encrypted nor
authenticated. Use of a cryptographic authentication mechanism for
all control/measurement messages is RECOMMENDED to mitigate this
concern, and would also provide protection from off-path attacks.
There is a design trade-off between the cost of introducing
cryptographic security for control messages and the desire to protect
control communication. For some deployment scenarios the value of
additional protection from DoS attack will therefore lead to a
requirement to authenticate all control messages.
Transmission of network control messages consumes network capacity.
This control traffic needs to be considered in the design of a
Circuit Breaker and could potentially add to network congestion. If
this traffic is sent over a shared path, it is RECOMMENDED that this
control traffic is prioritized to reduce the probability of loss
under congestion. Control traffic also needs to be considered when
provisioning a network that uses a Circuit Breaker.
The Circuit Breaker MUST be designed to be robust to packet loss that
can also be experienced during congestion/overload. Loss of control
messages could be a side-effect of a congested network, but also
could arise from other causes Section 4.
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The security implications depend on the design of the mechanisms, the
type of traffic being controlled and the intended deployment
scenario. Each design of a Circuit Breaker MUST therefore evaluate
whether the particular Circuit Breaker mechanism has new security
implications.
8. IANA Considerations
This document makes no request from IANA.
9. Acknowledgments
There are many people who have discussed and described the issues
that have motivated this document. Contributions and comments
included: Lars Eggert, Colin Perkins, David Black, Matt Mathis,
Andrew McGregor, Bob Briscoe and Eliot Lear. This work was part-
funded by the European Community under its Seventh Framework
Programme through the Reducing Internet Transport Latency (RITE)
project (ICT-317700).
10. Revision Notes
XXX RFC-Editor: Please remove this section prior to publication XXX
Draft 00
This was the first revision. Help and comments are greatly
appreciated.
Draft 01
Contained clarifications and changes in response to received
comments, plus addition of diagram and definitions. Comments are
welcome.
WG Draft 00
Approved as a WG work item on 28th Aug 2014.
WG Draft 01
Incorporates feedback after Dallas IETF TSVWG meeting. This version
is thought ready for WGLC comments. Definitions of abbreviations.
WG Draft 02
Minor fixes for typos. Rewritten security considerations section.
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